Imaging devices, such as imaging photometers and other types of digital cameras, generally include a lens assembly, a shutter, and an image sensor (e.g., a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device). In operation, an imaging device is positioned at a desired location relative to a light source (e.g., a visual display) to be measured and a user activates the device to capture an image measurement of the light source for a desired exposure time. During each individual exposure period, the lens assembly, mechanical shutter, and image sensor are all aligned along an optical axis and light from the light source can pass along the optical axis to impinge upon the image sensor.
The traditional type of mechanical shutter in many conventional imaging devices is an iris shutter. Such shutters use two or more blades that overlap each other and open apart and close together at a desired exposure rate to expose portions of the image sensor to the light from the light source being imaged. One drawback with this type of shutter, however, is that it generally exposes different parts of the image sensor for different lengths of time. For example, the center portion of the image sensor is generally exposed for a longer time than the outer portions of the image sensor as the shutter blades move outwardly from a closed position to an open position. The difference in exposure times can introduce significant errors into the image measurement. For example, in conventional iris shutter systems with a desired exposure time of 100 milliseconds, the shutter blades can take 5 milliseconds to completely open for the exposure, and another 5 milliseconds to completely close again at the end of the desired exposure time. The opening/closing times can accordingly introduce up to a 10% error in the image measurement because different portions of the image sensor are exposed to the light source for different periods of time. In order to minimize this error, it is often necessary to use relatively longer exposures and, accordingly, systems with iris shutters can be unsuitable for many applications where short exposure times are desired. Another drawback with iris shutters is that such systems include many moving parts that are constantly rubbing against each other and generally require a significant amount of maintenance and/or replacement.
Conventional imaging devices can also include an electronic shutter. This type of shutter operates by constantly flushing the detectors or pixels of the image sensor so that the sensor does not hold an electrical charge. During an exposure period, the flushing process is paused and an electrical charge is allowed to build up until the end of the desired exposure time. The charge is then discharged and the values are read by a suitable controller. Although electronic shutters do not generally have many of the above-described problems that plague systems including iris shutters, many types of high performance CCDs cannot utilize electronic shuttering and require mechanical shutters. Accordingly, there is a need to improve the mechanical shutter systems for high performance imaging devices.